Closing off access to the mail server on port 25 will definitely stop that from happening.
If you have a firewall between your mail server and your ceilent machines then block port 25. The Exchange server sends to other email servers using 25, but Outlook uses a different protocol.
If you dont have a firewall between your mail server and your clients, which most people dont, then you can change the configuration of the 'Virtual SMTP Server' in the Exchange System Maanger.
Go to System Manager > Servers > servername > Protocols > SMTP and bring up the properties for default.
Now go to the Access tab and then connection. Here you can block certain IP addresses but allow everything else OR allow some addresses and block all others. What you do here is going to depend on your configuration, what IP address space you are using, and how email finds it way to your exchange server.
The safest thing is probably to set 'all execpt the list below' and just block the offeding subnet from accessing the SMTP server.
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